Consider two absorption spectroscopy based techniques:
1- Direct absorption spectroscopy (DAS) in which the laser wavelength is tuned by 10 Hz.
2- Wavelength modulation spectroscopy (WMS) in which a fast (10kHz) wavelength modulation is added to the slow scanning (10Hz).
We know that with a similar optical setup, WMS has better (~100 times) sensitivity because of its remarkable noise rejection.
Can we say, compared to DAS, in WMS noise is rejected twice? First, because of the fast modulation which reduces the flicker noise and second, because of the lock-in amplifier which rejects noise from all harmonics other than 2f (and 1f)?
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WM => modulation of laser to a set of wavelengths and in turn frequencies Spectroscopy - Is dealing with measurement
To the bridge the two => generation of harmonics ( > f and 2f) or reduce the flicker noise . In this case the modulation of wavelengths that generates frequencies avoid the noise sources (from lower range) . With mode-locking to a range of harmonics you are eliminating the unwanted frequencies .
In comparison to DAS , the key here (WMS) is to tune the laser closer to absorption spectra of the sample rather than fixed step modulation.
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